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Topic: help with LED light bar?? (Read 3733 times)

Hey everyone Iím new to this forum and from reading other topics there seem to be many pros out there that really know there stuff about electronics so I figured this would be the best place to seek an answer to my question.

Now I have an electronics background and found it very easy to follow the schematic but I wish to modify it. What I would like to do is replace the two LED's in series on both the channels with 25 LED's in parallel for each stereo channel. What I donít understand is, is the 220 ohm resistor before the LED's there to supply the correct amount of voltage to the LED's and if so, what resistor values would I need for 25 LED's in parallel?

Also because I want to put them in parallel, would I need a resistor before each LED or could I just use one in front of the power supply/in front of the individual Light Bars??

I am using White LED's with a blue tint rated at 3.3 Volts and 20mA. I will also use a 12 volt power supply @ 1000mA and might make a voltage regulator to drop the voltage down to a further 3 Ė 4 Volts because Iím thinking that if I supply the correct amount to voltage to the LEDís I wonít need any resistors??

I would greatly appreciate any help from you guys and gals, I think this is really simple to work out with Ohms law but I just canít wrap my head around the problem.Cheers!

You will always need resistors to limit the current to the LEDs since they dont actually have their own resistance pretty much(They do but its very very very minimal. Almost the same as a regular wire).Using a 12v power supply with 25 LEDs, (id use 24 due to the numbers we're working with here, really. Ill continue as if using 24 LEDs) you would have a series-parallel circuit. 4 LEDs in series with 6 sets of 4 in parallel. That way, each of the 6 parallel branches will handle the full 12 volts. In each of the 6 branches, youll want a 150ohm resistor at the lowest in series with the 4 LEDs. That will supply 80mA at 12v to the 4 LEDs in that branch: ---R---L---L---L---L--- <-- like that. So you would basically have that little example circuit 6 times in parallel with eachother. Then on the common leads, connect the positive side to the Collector of a power transistor, and the other side to GND. Then connect the Emitter to your 12v power source. Next hook the Base up to the music input and you should be good to go.

Its almost 4AM so my transistor hook-up may not be 100% correct so wait for someone to confirm or shoot down my advice on this one. At this point in time, however, im fairly certain my thoughts here are sound.

Im assuming your doing this in a car/truck?

-EDIT-Oh, and those 220ohm resistors are there to supply the proper current, not the voltage. Since LEDs have no measurable resistance of there own, a resistor needs to be added to drop the current. The instructable setup has 2 LEDs that draw 20mA so with 2 in series they draw 40mA. At a source voltage of 9v, a resistance of 220ohms would supply a current of 40.1mA to those 2 series LEDsIn your situation, if you put all of your LEDs in parallel, they would all fry instantly. They would each have 12 volts running through them when theyre rated for only 3.3v.

yes dragon is correct, i myself have been looking for something like this for a while and up until that instructible was made there were only 2-3 and all barely explained what i wanted to know. this one is great!i attached a schematic for you to use.

ahh, my bad... just to reference my previous post, the leds are back to front.... they would be hooked up with the positive leg toward the resistor. ive recently read a few schematics that a friend drew up and the leds were backward which kind of confused me sorry.

There are some chips out there that can limit the current internally so no resistors at all are needed. Take the TLC5940/5947 for example, you only need one resistor to set the current of all of the LEDs.

Thank you everyone who has replied, sorry for not thanking you earlier, I wasnít getting email alerts to let me know people where replying. Oh and SMASH, that schematic is great; would I just replicate that for the second channel of my schematic?

Also if anyone has any good ideas as how to lay the schematic for the LEDs into a "bar/tube" configuration using prototyping board with holes/long lines of copper track so that I can use them as light bars. It needs to be narrow and I do not have the equipment to etch my own PCBs.

i am fine with making my own light bar, what i dont understand about the schematic is how the voltage and current is divided from the power supply to the two seperate channles in the circuit. does each side recieve the sum of the power supply and its potential current (eg - power supply is 5 Volts @ 1000mA, do both sides of the schematic recieve 5 Volts and a potential of 1000mA) or do both side recieve an eaqual share of the voltage and current (eg - 5 Volt Power Supply @ 1000mA, does each side recieve 2.5 Volts and a potential of 500mA (also how does the voltage behave when one side of the circuit is on and the other off?))

Both the left and right halves of the light are connected in parallel to the battery. So basically, you can imagine them as two separate circuits that are just powered by the same battery. The battery will just give as much current as each circuit needs.

well, i pmed you but meh... yes to the copper track question.get some protoboard with tracks not individual holes and have all +'ivs on one side and all -'ivs on the other.each side gets full voltage if the channels are wired in parallel, if in series the voltage drops.if you have a 1000Mah supply your circuit can draw 1000mA for 1hr, 2000mA for 1/2 hour and so on...

Thanks for your help everyone. I have sorted out my resistors. I will be using a 75 ohm resistor in series with each LED which will be connected in parallel. All I need to do know is figure out how to lay it all down on a PCB. SMASH gave me an idea so I sketched the idea in paint and attached the picture to this post. Check it out and see if it will work. I am trying to make the layout as thin as possible and I think I have got it to be as thin as it will go. I am not setting the resistors in vertically because I donít want them getting in the bay if the LEDs so I have laid them flat.

You could do without a PCB, just make a long piece of acrylic or similar, drill holes to glue the LED's into and wire it up (thin enamelled solder-through wire would make it a quick job). Cut one pin of the resistors short, so it can be soldered directly to the LED and Bend the other pin in a right angle holding the part right at the resistor with a thin plier, so as not to damage the pin-to-resistor connection. Then you can solder the free ends of the resistors to each other one by one.

Logged

Regards,SÝren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?Please remember...Engineering is based on numbers - not adjectives

thanks for the idea. i had thought of something similar to what you mentioned but i didnt want to fiddle around with bending LED and resistor leads. i am going to lay them out on a vero style PCB, cut it to width and slide it into an acrylic diffuser used in LED bar lights that are designed to replace flurecent tubes. i hope i get it all to work.cheers.

Wow I have NO idea what the heck I did with my math there.... Thanks for picking up my slack Soeren. Sorry Rev for giving false info.

No prob. I thought that you may have made a mistake because i was so confident with the info that I have learnt from this forum. Everyone makes mistake and it is so easy to do when working with conversions such as milli amps, amps...